1. Field of the Invention
The present invention relates to a control switch for an automatic vehicle transmission and, more particularly, to the control switch which generates, one at a time, position signals indicating respective gear positions of a gearshift lever employed in an automatic vehicle transmission.
2. Description of the Related Art
An automatic vehicle transmission is generally provided with a gearshifting manual shaft that is driven by a gearshift lever. A control switch for detecting the position of the transmission is arranged in the vicinity of the manual shaft so that an electric position signal indicative of one of the gear positions selected by the gearshift lever can be generated therefrom. The position signals that can be generated by the control switch one at a time may include those associated with, for example, parking, reverse, neutral, drive, second, low or the like. The position signal is supplied to a microcomputer of a control system to allow the latter to control various displays and also to control the status of the transmission.
The control switch of the above-described type comprises a housing 51 consisting of a case 51a formed by die-casting aluminum and a base substrate 51b molded of a synthetic resin, as shown in FIGS. 14A to 16. The housing 51 is mounted, as illustrated in FIGS. 17A and 17B, on the transmission 50 with the base substrate 51b held in contact with the transmission 50. A movable body 60 is rotatabley accommodated inside the housing 51 and coupled with a manual shaft 41 that is rotatable in response to movement of a gearshift lever. Therefore, the movable body 60 rotates together with the manual shaft 41 and, hence, in response to the gearshifting of the gearshift lever.
The movable body 60 has a tubular boss 61, to which the manual shaft 41 is coupled for movement together therewith, and a movable element 62 coupled with one point of an outer circumferential face of the tubular boss 61 so as to project in a diametrical direction. The movable element 62 turns following the turn of the tubular boss 61 within a space defined by a recess 52, formed in an inside face of the case 51a, and the base substrate 51b. A plurality of movable contacts 63 are set on a face of the moving element 62 opposed to the base substrate 51b. The movable contacts 63 are urged towards the base substrate 51b by springs 64. Moreover, substantially arcuate band-shaped fixed contacts 54a and 54b are arranged at the base substrate 51b about the center of turn of the movable body 60. The fixed contacts 54a correspond to respective shift positions such as parking, reverse, etc., and the fixed contact 54b is common to all of those fixed contacts 54a and engageable with any one of those fixed contacts 54a as will be become clear from the subsequent description.
When the movable body 60 turns, the common contact 54b is electrically connected with one of the fixed contacts 54a via the movable contacts 63. The position to which the movable body 60 has been turned can be detected in terns of which one of the fixed contacts 54a is connected with the movable contacts 63. In other words, when the manual shaft 41 in the transmission 50 is turned in response to gearshifting of the gearshift lever, the movable body 60 turns together with the manual shaft 41 with the movable contact 63 consequently bridging between one of the fixed contacts 54a and the common contact 54b to thereby generate a position signal corresponding to the position of the shift lever in the transmission 50. In the illustrated example, when the movable contacts 63 are in position to connect one of the fixed contacts 54a, corresponding either the parking position or the neutral position, and the common contact 54b, a power feed route is formed from a battery to a starter circuit through an ignition switch.
The transmission 50 is disposed in an engine room or at a lower part of au automobile body (or under the body) and consequently the control switch fitted in a manner to butt against ATM 50 is exposed to waiter, mud, dust, etc., and the control switch is thus required to be waterproof Meanwhile, the control switch tends to be heated to 100.degree. C. or higher as it is mounted in a fashion held in contact with the transmission 50, whereby a high atmospheric pressure difference is brought about between inside and outside the casing 51 if the housing 51 is sealed, which acts a stress and easily breaks the housing 51. In order to prevent the atmospheric pressure difference, a ventilation hole 70 is formed in the housing 51.
The ventilation hole 70 is formed in the case 51a, which is placed at an upper part of the housing 51 when the housing 51 is mounted on the transmission 50, with an aim to avoid the water and mud. The reason that the ventilation hole 70 is formed in the case 51a is because it is more reliable about waterproof properties to provide the hole 70 in the case 51a made of a metal than to provide the hole 70 in the main body substrate 51b made of a synthetic resin. Moreover, the ventilation hole 70 is equipped with a filter 71 (See FIG. 8A and 8B) using a waterproof gas-permeable material (e.g., material by the trade name GoreTex), thereby being water-resistant. If the housing at a high temperature is suddenly cooled by water, a negative pressure would develop inside the housing 51 to such an extent as to allow water to be sucked into the housing 51. The filter 71 is set to the ventilation hole to prevent the water from entering even under the above situation.
The case 51a in which the ventilation hole 70 is formed is metallic and constructed in a structure shown in FIGS. 18 and 19 to hold the filter 71. More specifically, a metallic cylindrical body 73 is inserted into a cylindrical part 72 which is provided in the case 51a in communication with the space between the recess 52 of the case 51a and the base substrate 51b, thereby holding the filter 71. A rubber seal 74 is interposed between an inner circumferential face of the cylindrical part 72 and an outer circumferential face of the cylindrical body 73 to water-tightly couple the cylindrical part 72 and cylindrical body 73. The cylindrical body 73 has a step part 73a at one end part thereof The step part 73a is of a larger diameter at the side of an opening of the cylindrical body 73. A washer 75 is fitted to the step part 73a via the filter 71. The filter 71 is secured to the cylindrical body 73 via the washer 75 by compressing the opening of the cylindrical body 73. Further, a step part 72a is formed at an opening part of the cylindrical part 72, which has a larger diameter at the side of the opening part. A protecting cap 76 is pressed into the step part 72a.
In the above-described constitution, since the ventilation hole 70 is opened to the side opposite to the transmission 50, the ventilation hole 70 is exposed outside when the housing is mounted to the automobile and prone to be affected by water and mud. In spite of the waterproof structure as discussed hereinabove, an insulation failure is highly probably brought about due to the invasion of water into the housing 51. If the water enters the housing 51, it is difficult to discharge the water outside the housing 51 because the ventilation hole 70 is located at the upper part of the housing 51.
Besides, the ventilation hole 70 requires the cylindrical body 73, rubber seal 74, washer 75 and protecting cap 76 in addition to the filter 71, that is, five parts in total. In other words, the increased count of parts invites a cost rise. The metallic cylindrical body 73 is partly caulked so as to secure the washer 75 holding the filter 71 and therefore, waterproof efficiency between the washer 75 and filter 71 is varied. Whether or not the filter 71 is held enough at the periphery thereof cannot be confirmed directly. Although an air pressure can be impressed to the filter 71 in order to confirm the water-tight properties at the periphery of the filter 71 in a state fitted to the cylindrical body 73 thereby shutting an opening part of the washer 75, the washer 75 comes to tightly adhere to the filter 71 when the opening part of the washer is pressed strong even if the cylindrical body 73 is not caulked enough. In that case, the ventilation hole is incorrectly recognized as good during inspection. Two waterproof structures required by the ventilation hole to the filter 71 and rubber seal 74 decrease reliability in waterproof performance. The rubber seal 74 particularly accompanies a creep because of heat, thereby lowering the waterproof efficiency of the ventilation hole.